1. Field of the Invention
The present invention concerns molding optical lenses, especially, but not exclusively, ophthalmic lenses, from a polymerizable synthetic material.
2. Description of the Prior Art
The traditional mold comprises two molding shells at the periphery of which is disposed an annular closure member defining with them the required molding cavity.
If the two molding shells are made of glass and have substantially parallel surfaces, for example, the closure member is a seal in which the molding shells are nested or a sleeve which surrounds them.
If, as described in U.S. Pat. No. 5,110,514, for example, the molding shells are more or less massive, the closure member can be at least in part in one piece with one or both molding shells.
The conventional way to fill a mold made up in this way of two molding shells and a closure member is by causing the molding material to flow into the molding cavity through a casting opening provided for this purpose in the closure member. This opening is at the periphery of the molding shells so as not to interfere with the optically active area of the optical lens formed.
In a manual process the molding shells can be more or less flat, i.e. substantially horizontal, when the material is cast.
In an at least partly automated process, in which the mold to be filled is vertically aligned with a filler device adapted to deliver a particular quantity of molding material through a nozzle, the molding shells are disposed on edge, i.e. substantially vertically, and this is also the case, for specific reasons, in the U.S. Pat. No. 5,110,514 already referred to.
When the molding shells are disposed on edge in this manner, the mold is filled from the top by gravity only.
Experience shows that unless special measures are implemented, for example a relatively long waiting time (in the order of two hours, for example) between filling the mold and starting polymerization, the optical lenses obtained frequently have localized optical defects of greater or lesser severity, leading to relatively high reject rates.
The present invention is based on the surprising observation that these optical defects can largely be avoided if the mold is filled from the bottom, this going against the process which otherwise seems the most natural in that it benefits from the effect of gravity.
The explanation may lie in the fact that, with filling from the top, the "syrup" effect well known to glassmakers is inevitably operative, at least to some degree: given its viscosity, the molding material flows slowly, in "runs", into the molding cavity and there is no close control over this process; also, the molding material is a mixture of constituents which can have different refractive indices and/or coefficients of viscosity, so that stretching or shearing can cause relatively large local index gradients which inevitably produce the optical defects noted if polymerization occurs too soon after filling.
A long waiting time between filling and starting polymerization is then required for good homogenization of the material, in order to avoid optical defects.